You can start by looking on popular fanfic websites like FanFiction.net or Archive of Our Own (AO3). These platforms have a wide variety of Naruto - related fanfics, so you might be able to find some specifically about black chakra there.
Another option is to check Naruto - focused fan forums. Often, members of these forums share and discuss their own fanfics or ones they've found. They may have a section dedicated to unique concepts like the black chakra in Naruto fanfics.
One interesting 'Naruto Black Chakra' fanfic could be one where the main character has to learn to control this black chakra in a unique way. Maybe they discover that it has powers that can be used for good instead of being feared. Another might focus on the origin of the black chakra, exploring some ancient ninja lore.
I'm not sure if there are extremely famous 'naruto chakra gun' fanfics. But it's possible that in the vast sea of Naruto fanfics, there are some that have gained a following due to this unique concept. Some fans might be really drawn to the idea of a chakra gun and so might promote or share those fanfics more often. You could also check Naruto - related forums to see if anyone has recommended such fanfics.
There's a fanfic named 'Chakra Genesis'. In this story, Naruto has the ability to generate new types of chakra from his emotions. For instance, his determination creates a 'resolute chakra' that gives him enhanced strength and speed. The way the author plays with the concept of chakra manipulation based on emotions is really unique. Another great one is 'Naruto's Chakra Kaleidoscope'. It's like the Sharingan's abilities but for chakra manipulation. Naruto can see different chakra patterns and use them to his advantage in battles, which is a very original idea.
The black chakra in Naruto fanfic could potentially be related to some dark and ancient power. Maybe it has a connection to an old curse or an evil entity that has latched onto Naruto. It could also be the result of some experimental jutsu gone wrong in the past that has now manifested as this black chakra within him.
Naruto's chakra projectile can also evolve in relation to his relationships. If he teams up with a new ally who has a special chakra - related ability, they might combine their powers. His chakra projectile could then incorporate the ally's chakra signature or ability. For instance, if he partners with someone who has ice - based chakra, the projectile could have an icy shell around it, increasing its destructive power and giving it new tactical uses on the battlefield.
In some fanfics, Naruto's chakra chains could be a result of some ancient Uzumaki clan ability that he discovers. Maybe it was a hidden power within his bloodline all along, and through intense training or a particular encounter, he unlocks this ability to use the chakra chains.
In some Naruto black chakra fanfiction, the black chakra might赋予 characters unique abilities. It could enhance their strength and speed exponentially, but at the cost of their sanity. This creates a really cool internal struggle for the characters as they try to control this powerful and dangerous force.
One common plot device is that Naruto is able to unlock new and powerful forms or jutsus because of his potent chakra. For example, he might develop a unique transformation that only his large chakra reserve can sustain. Another is that he becomes a target for powerful villains who want to either steal his chakra or use him for their own purposes.
Sasuke is often a character associated with black chakra in fanfiction. Given his natural inclination towards power and his dark past, it's easy to imagine him being connected to such a powerful and mysterious form of chakra.
In some fanfictions, Naruto's black chakra might represent a new, darker side of his power. It could be related to some sort of ancient or forbidden energy. For example, it might give him the ability to access shadow - like techniques that are more in tune with the darker aspects of the ninja world, like stealth and silent assassination skills that normal Naruto doesn't typically use.